Hi John. You can usually look directly at the ram and read the print on the chips themselves. It will sometimes say "DDR" somewhere in the description. Also, look at your BIOS, sometimes it will tell you in there as well. Another way to tell is find a stick of 1 or the other (either old pc133/100 or DDR) that you KNOW is that type, and try to plug it into the slot your existing stick is in. If it goes in, that's the type. They aren't backwards compatible slots.